Biosynthesis of Milk

Introduction

Mammary epithelial cells in the mammary gland synthesize complex milk constituents from simple components present in circulating blood. The mammary gland is equipped with an extensive network of arterial and venous blood capillaries. The components of blood needed for milk biosynthesis are extracted from arterial blood. To produce 1 litre of milk 500 ltr. of blood has to pass through the udder. The venous blood carries the blood back to the heart for recirculation and component renewal. Based on the percentage of precursor difference in arterial and venous blood, the ability of mammary gland to extract milk precursors from arterial blood is remarkable, in that it could approach as high as 20 L/min.

The ratios of major milk and blood components suggest that fat, sugar, potassium, calcium, magnesium, and phosphate are several times more abundant in milk than in blood. For instance, compared to blood, the sugar content of milk is 90 times, calcium content is 13 times, phosphorus content is 10 times, fat content is nine times, and potassium content is five times more. Fat and protein are in colloidal dispersion; fat as emulsified globules with membranous coating and proteins as micelles. The minerals, vitamins, and lactose are in true solution form. For biosynthesis of milk constituents, the primary substrates extracted by mammary epithelial cells from their counterparts in blood include glucose, amino acids, fatty acids, β-hydroxybutyrate, and salts.

Biosynthesis of milk proteins

In the ruminant animals, all food must pass through rumen prior to digestion in the stomach and intestines. A large proportion of dietary protein is transformed by rumen bacteria and protozoa, thereby generating high quality microbial protein with significantly better amino acid profile than that of the vegetable protein in the feed. After digestion, the microbial protein along with smaller quantity of feed protein (that escaped rumen digestion) gives rise to small peptides and amino acids. These are then transported across the intestinal wall into blood, which ultimately form the substrate for protein synthesis in mammary gland.

The substrate, amino acids from the blood, is transported through the basolateral membrane to mammary secretory cell. The transporting systems may be sodium dependent or independent. Milk proteins are encoded by specific genes in the genome. The biosynthesis is initiated by gene expression, which itself gets initiated by the hormone-induced transcription factors.

1. Transcription occurs in the cell nucleus. It involves the formation of messenger RNA, which carries the code of a specific protein. The mRNA is assembled in ribosomes attached to the rough endoplasmic reticulum (RER).

2. Activation of amino acids in the cytoplasm takes place by reaction with ATP and subsequent attachment to transfer RNA (tRNA). Each tRNA is specific for an amino acid.

3. Translation occurs in the ribosome. The code for amino acids resides in mRNA. Each code comprises of three nucleotides called codon. A trinucleotide called anticodon is contained in the tRNA, which recognizes the codon. Each codon comes into position and appropriate amino acid-tRNA complex is added to form peptide chain.

The polypeptide chain then folds up in a configuration dictated by the physical forces inherent to the sequence of the amino acids. Other groups like phosphates of calcium, as in case of casein, are added later. Finally, the protein assumes its three dimensional structure that gives the protein its distinctive function. Following synthesis, milk proteins being secretory proteins are transported from the cell into alveolar lumen to merge with other milk constituent pool.

Total nitrogen distributed among various fractions is; caseins (76%), whey proteins (18%), and non-protein nitrogen (6%). Casein exists as calcium phosphate complex in the form of colloidal suspension, while whey proteins occur in soluble form. Another type of milk protein occurs as a part of milk fat globule membrane, covering the envelope in which milk fat is enclosed. Casein molecules in milk occur as spherical particles called micelles.

Some milk proteins originating directly from blood enter mammary gland via plasma cell adjacent to the secretory epithelium and are spilled into milk unchanged. Milk also contains various enzymes derived from biosynthetic activity. Urea, creatine, and creatinine are non-protein nitrogen compounds, which originate from blood as well. Minerals of milk are derived from blood and their level is determined by Donnan equilibrium and osmotic conditions.

Biosynthesis of milk lipids

Milk fat, in freshly secreted milk, occurs as microscopic globular emulsion of liquid fat in aqueous phase of milk plasma. The functional properties of milk fat are attributed to its fatty acid make up. More than 400 distinct fatty acids have been detected in milk. Typical milk fat consists of 62% saturated, 29% monounsaturated, and 4% polyunsaturated fatty acids. It contains 7–8% short-chain fatty acids (C4–C8), which is a unique characteristic of milk fat. Milk fat functions as a concentrated source of energy as well as a source of fat-soluble vitamins A, D, E, and K and essential fatty acids—linoleic and arachidonic acids. Cholesterol is a component of blood from where it enters milk pool.

The fatty acids needed in the synthesis of triacylglycerol (triglycerides) come from two sources described below.

1. Blood plasma lipids originating from digestion and absorption of dietary fat as well as by mobilization from adipose tissue. Approximately, 50% of fat fatty acids of milk owe their origin to blood lipids. In this regard, most of the C18 fatty acids and about 33% of C16 fatty acids originate from dietary fat.
2. De novo synthesis in the mammary epithelial cells utilizes acetate (C2) and β-hydroxybytyrate (C4) as sources of carbon. Nearly, all C4 to C14 fatty acids are synthesized from these two precursors.The acylglycerols or glycerides of milk are synthesized in the cytoplasm surface of the smooth endoplasmic reticulum of mammary epithelial cells, employing a key enzyme Acetyl CoA carboxylase. This enzyme becomes very active during lactogenesis. Milk lipids are synthesized via α-glycerol phosphate pathway. Two acyl CoA molecules react with α-glycerol-3-phosphate to form phosphatidic acid, which converts to 1, 2 diacylglycerol upon removal of the phosphate. An additional long-chain acyl CoA adds the final fatty acid to form the triacylglycerol and CoA.

Biosynthesis of milk sugar (Lactose)

Glucose is the exclusive monosaccharide substrate for lactose biosynthesis. In ruminants, 45–60% of blood glucose is formed from propionate in the liver by Gluconeogenesis process.

In the bovine mammary gland, 60–70% blood glucose is used for lactose synthesis. Two molecules of glucose give rise to one molecule of lactose. Glucose is converted to UDP-galactose by a cascade of several enzymatic reactions. At the onset of parturition, the enzyme activity shows a dramatic increase to cope up with lactogenesis (the lactation process).

Glucose and UDP-galactose are combined to form lactose, catalyzed by the action of lactose synthetase that is composed of galactosyl transferase and α-lactalbumin. The rate of lactose biosynthesis is determined by the availability of α-lactalbumin from the RER.

Secretion of milk constituents into the lumen

Milk constituents are individually synthesized inside the secretory cell. After they are transported to lumen space, they blend together to form so called milk. The process for secreting nonfat constituents differs from that of milk lipids. Milk proteins synthesized in the RER are incorporated into Golgi vacuoles or vesicles along with lactose and minerals. The secretory vesicles then separate from the Golgi apparatus and transport molecules towards the apical region of the cell. The membrane surrounding the vesicles fuses with the plasma membrane of epithelial cells followed by delivery into lumen space.

Milk lipids follow a discrete secretory process. As the molecules of synthesized milk fat transfer from the endoplasmic reticulum toward the apical membrane, their droplets grow in size. While passing through the apical membrane, they are pinched off as spherical globules with a coating of apical plasma membrane. The fat globule membrane forms an envelope around fat particles.

Milk fat globule membrane (FGM): The fat globules are stabilized by a very thin membrane, closely resembling plasma membrane. The FGM is only 5–10 nm thick. The fat globule membrane consists of proteins, lipids, lipoproteins, phospholipids, cerebrosides, nucleic acids, enzymes, trace minerals, and bound water.